If two people share part of the same DNA segment with me, will the centiMorgan value for each segment be proportional to how long the segments look?

Overlapping DNA Segments

cM

Matching on Chromosome 1

Match 1

Match 2

Non-Match

20.5

7.6

No. Part of a matching DNA segment may have a higher or lower centiMorgan (cM) value than the segment’s average. One of your matches will then have a much higher cM value for their segment.

cM Values For Three SNP Sets

SNP Set

cM

SNPs

Chromosome

Start Location

Stop Location

221

0.48

100

1

82521165

82870559

222

0.26

100

1

82871547

83219651

223

0.48

100

1

83221230

83764336

This is because the recombination rate across the human autosomal genome is highly variable. For example, one matching SNP set might be 349,394 bp (base pairs) long and .48 cM while the one next to it is about the same length but is only .26 cM.